This invention relates to a liquid crystal device, more particularly relates to a liquid crystal device with a charge storage structure.
Liquid crystal devices have been employed for the displays of microcomputers, word processors, television systems and so forth, because of their high contrast indication ability. On the other hand liquid crystals have been considered promising as competitive media for memory devices such as disc memories and for applications to such audio-instruments as speakers.
In prior art, there have been known such a smectic liquid crystal device which comprises a pair of substrates opposing to each other with a liquid crystal layer therebetween, a pair of electrodes provided on the opposed insides of the substrates and also a pair of oriented films symmetrically provided on the electrodes, where a simple matrix structure or an active element structure is constructed with non-linear devices. A very important characteristic required of such a liquid crystal device is a large coercive electric field Ec (threshold electric field). The large Ec makes it possible to hold the liquid crystal layer at a certain condition, for example, when the electric field applied to the layer is removed. And, when the applied electric field increases to a level beyond the strength, the liquid crystal layer changes abruptly its condition into a transparent condition, and vice versa. In this regard, the coercive electric field Ec takes a positive value Ec+ (a threshold value observed when applied with a positive voltage), and a negative value Ec- (a threshold value observed when applied with a negative voltage). Although Ec+ and Ec- are not necessarily the same, both can be about equal by virtue of a processing condition of an orientation treatment on surfaces contiguous to a liquid crystal layer.
However, such a smectic liquid crystal layer exhibits only a very faint hysterisis, namely, has a very small and unstable Ec. Especially, when a surface stabilized ferroelectric liquid crystal layer is designed, the value of Ec depends largely on the strength of pulsed electric field applied to the layer. Hence, an excitation system known as AC bias method has been employed in which a negative pulse signal is applied in advance of rewriting in a positive direction, and then a positive pulse is applied under a fine control in terms of the strength of electric field and the applying time, and in inverse, when rewriting in a negative direction is desired, procedure is taken in a same manner as the former case where the direction of electric field is reversed. The AC bias method makes a circumferential circuit very complicated.
Accordingly, a need exists to make a liquid crystal device having a circumferential circuit less complicated than that when using as AC bias method. On the other hand, the bias method seems indispensible for implementing a liquid crystal device as long as a liquid crystal layer has only a slight Ec. There have been other attempts to make a liquid crystal layer with a stable Ec. However, they are compromising to its frequency characteristics.